In a mobile communication system, when a call is placed to a terminal (paging terminal), it is necessary to inform the terminal that the call is placed to the terminal. The network of the mobile communication system manages the location information of each terminal as the location registration area of the terminal by allocating a Location Area Identifier (LAI) for each location registration area. The network informs each of the terminals in the location registration area where the paging terminal is registered that the call is placed to the paging terminal. This procedure is called paging (see, for example, Non Patent Document 1).
FIG. 1 shows a procedure of receiving paging information in a W-CDMA system.
As shown in FIG. 1, first, the base station transmits a paging indicator channel (PICH) (step S2).
Next, the base station transmits a paging channel (PCH) (step S4). After receiving the PICH, the mobile station determines whether a group including the mobile station is being paged. When determining that the group including the mobile station is being paged, the mobile station receives the PCH from the base station.
After receiving the PCH, the mobile station determines whether the mobile station is being paged. When determining that the mobile station is being paged, the mobile station (paging mobile station) transmits a random access channel (RACH) (as a response to the received PCH) (step S6). Namely, the paging mobile station does not respond to the PCH by transmitting the RACH until the paging mobile station receives the information indicating that the mobile station is being paged (paging information).
After receiving the RACH as the response to the PCH from the paging mobile station, the base station transmits information necessary for setting up a radio line to the paging mobile station (step S8).
Based on the received information necessary for setting up the radio line, the paging mobile station starts the procedure of setting up the radio line, and transmits an “ACK” signal to the base station (step S10), and starts communications (step S12).
In such a mobile communication system, as shown in FIG. 2, the paging information is simultaneously transmitted in each of the cells of the same paging area. Namely, in each cell of the same paging area, the paging information is transmitted through the PCH regardless of whether the paging mobile station is located in the cell.
Further, in such a mobile communication system, it is known that the mobile station reduces battery power consumption of the PCH by performing the following method.
As shown in FIG. 3, each mobile station is included in one (group) of a plurality of groups, and when the base station transmits the paging information through the PCH, the paging information is transmitted to each of the groups by using a time-division multiplexing method.
Each mobile station performs a receiving operation only when the paging information is transmitted for the group including the mobile station. By doing this, the power consumption is reduced.
Further, each paging indicator channel (PICH) for the corresponding group may be provided so that the PICH includes the information whether there is the page information for the corresponding group. By doing this, each mobile station may detect whether the group including the mobile station is being paged, thereby avoiding the reception of unnecessary PCH.
Non Patent Document 1: “W-CDMA mobile communication system” pp. 255-256, edited by Keizo TACHIKAWA, published by Maruzen K.K.